Environmental Chemistry in the Polar Regions​

The 34th RSC Environmental Chemistry Group Distinguished Guest Lecture and Symposium took place in the Council Room of Burlington House on Tuesday March 6th 2007. Attendees from the UK and other EU countries listened to a programme of five lectures on the theme of environmental chemistry in the Polar Regions.

The symposium was opened by Brendan Keely (University of York) who spoke about sediment records of environmental change in Antarctica during the last 10 000 years. Although difficult to analyse, pigments preserved in Antarctic lake sediments can be used as biological markers to provide information about source organisms. The talk focused particularly on photosynthetic pigments (chlorophylls derived from algae) since these are biological markers which relate exclusively to primary producer communities. The analysis and identification of such pigments are challenging because they are present in complex distributions of defunctionalised forms. However, by using multiple diode array high-pressure liquid chromatography (HPLC) and multistage tandem mass spectrometry (MS) (seven stages), a great deal of information was obtained.

Results from the Larsemann Hills region in East Antarctica (the low altitude lakes Pup Lagoon and Kirisjes) showed the presence of the farnesyl functional group, suggesting that the chlorophylls present were predominately those from anoxygenic bacteria. Additionally, results from different sediment layers also revealed changes in alkylation at C8 and C12, which provide a record of changes in ambient light intensities. Interpretation of such data led to an understanding of the way in freshwater dominance gave way to an anoxic 300 year marine inundation (6525-6205 BP), which itself gave way to a further freshwater system. This level of detail about Holocene sea level change in Antarctic contributes significantly to the understanding of natural variations in global climate.

Tim Jickells (UEA) then spoke on how productivity in Antarctic waters is controlled. Primary productivity is dominated by microscopic phytoplankton (the basis of the oceanic food chain and a significant global carbon dioxide sink) which are regulated by the availability of light and nutrients (e.g. nitrite, nitrate, ammonium, dissolved inorganic phosphorus, and iron). Arctic waters are very productive which contrasts with the iron-depleted Antarctic waters which are not productive. This is because global dust fluxes (from the great deserts) are the natural sources of iron, and deposition in Antarctica is low. However, some areas of the Antarctic have relatively high productivity which is thought to be due to sediment iron supply from the Antarctic Peninsula (‘a filament of iron-rich water extending from the peninsula fertilising the waters before the iron precipitates out’). Temperature, salinity, light penetration and density also have a large effect on water column structure and productivity. The annual productivity of chlorophyll has a summer peak (December – April) and inter-annual variability is high. Seasonal cycles in the concentrations of ammonium, nitrate, silica and phosphates are marked.

Although there is recognition of its importance to marine (and global) food chains and although there is acknowledgement that climate change will have a profound effect, it is apparent that the understanding of the control of productivity in Antarctica is incomplete and that without support for further exploration of its complexities, it will remain so.

Following these two papers about biological activity in Antarctica, Anna Jones (British Antarctic Survey; BAS) turned the attention to tropospheric chemistry (Tropospheric Chemistry in the Polar Regions). Originally it was thought that the remote polar troposphere would be fairly chemically uninteresting with low mixing ratios of reactive trace gases (‘the Antarctic – remote and clean; the Arctic – less remote and clean’) hence it was thought that it would be a good place to study a clean background atmosphere. Anna Jones discussed two aspects of the tropospheric chemistry which challenged this assumption − tropospheric ozone depletion events (ODEs) and photochemical processes which occur in snow.

In the Polar Regions, tropospheric ozone was expected to follow a seasonal cycle with a minimum in summer and a maximum in winter. However, significant deviations from this expected behaviour are found. In coastal stations, such as the Alert Arctic Station and the Antarctic station Halley rapid ODEs have been observed. Back trajectory calculations show that these events are associated with sea ice. The species responsible has been identified as BrO arising from sea ice source such as sea salt aerosols or frost flowers. Springtime conditions of extensive sea ice, an appropriate wind direction and inversions can produce depletions which extend up to several kilometres in height and which last for several days. The understanding of ODEs makes an important contribution to the modelling of atmospheric chemistry in the Polar Regions and the understanding of regional climate.

Assumptions about chemistry at the poles included the belief that the snowpack was chemically inert. However, in the mid 1990s measurements in Greenland showed elevated levels of NOx within the snowpack. This was subsequently found to be due to the photolysis of nitrate present in the snow. As a consequence NO and NO2 are released into the boundary layer. The most extreme example of this photochemical effect is at the South Pole where emissions from snow (driven especially by the high UV radiation and high concentrations of nitrates in the snow) are concentrated into the shallow boundary layer resulting in the high levels of NO. This enhanced NO affects other chemistry, including OH chemistry.

Dwayne Heard (University of Leeds) complemented the environmental chemistry described by the previous speaker by focussing on the measurement of free radical concentrations in the Polar Boundary Layer − particularly OH radical concentrations and those of HO2, RO2 and the halogen oxide radicals, BrO and IO.

Because concentrations of radicals are low (for OH ~ 106 molecule cm-3), in situ fast and sensitive techniques are required. FAGE – which uses laser induced fluorescence to quantify OH and HO2, CIMS (Chemical Ionisation Mass Spectrometry) for OH, HO2 and RO2, and DOAS (Long Path Differential Absorption Spectrometry), for BrO and IO, have all been used in polar environments. The data obtained are then compared to predictions of box models using detailed chemistry, for example the Master Chemical Mechanism. The level of agreement with the model is a powerful indication of whether we understand the chemistry of the Polar Regions.

Data from various stations (Palmer Station Antarctica, Summit Station Greenland, South Pole and Halley) were presented together with model calculations. Hydroxyl radical concentrations at Palmer Station were low (average 1.1 x 105 molecule cm-3) and consistent with it being a remote pristine environment. The higher levels measured at Summit in Greenland and South Pole (up to a few 106 molecule cm-3) were consistent with observations of snow emissions of NOx, nitrous acid and formaldehyde, which accelerate the cycling of radicals and increase OH. For OH and HO2 measurements from Halley, agreement with model calculations was improved when the reaction of HO2 with IO and BrO was included, together with the photolysis of HOI and HOBr photolysis as sources of OH. The levels of BrO and IO were measured to be highest in oceanic winds when the air came from regions likely to contain frost flowers. The chemistry of OH at Halley in the summertime appears to be controlled by coupling with halogen chemistry, which was unexpected.

The 2007 Environmental Chemistry Group Distinguished Guest Lecture (Frozen in time: the chemistry of polar ice cores) was given by Eric Wolff (BAS). The study of palaeoclimates and palaeoatmospheres by examining the gaseous content of ancient ice cores is essential not only for an understanding of how climate (and the Earth) responds to different conditions and but also for the testing of models being used to predict future climate. Ice cores contain a record of ancient air often with well-dated annual resolution and (currently) they provide data on the earth’s atmosphere as far as 800 kyr (800,000 years) ago (this is from Dome C a drilling site 900 km inland from the Italian station at Terra Nova Bay). Drilling for cores takes place at a variety of sites. Vostok (the Russian Station) has provided cores with 420 kyr of ‘measurable’ ice. Dome A (1200 km from the China station at Zhongstan) is one of several sites which offers promise as a drilling site with data from a million years ago. However, there are problems with coring. Access to cores is geographically limited, their acquisition is expensive, and dating does become poorer in sites with a low snow accumulation rate.A variety of signals can be obtained from cores. The oxygen and hydrogen isotopic content of the water molecules provides information about temperatures at the time of snowfall; both soluble and insoluble species are trapped at the surface by falling snow and there is also dry deposition and gaseous uptake onto the surface (e.g. sulfur from volcanoes, biogenic sulfur from DMS); as the snow gets deeper, pressure turns the loose snow into solid ice with bubbles of trapped air containing a sample of stable gases from the atmosphere (e.g. carbon dioxide).

There is clear evidence that anthropogenic activities have increased methane, nitrous oxide and carbon dioxide over the last 200 years – and although natural variability in these species has been observed in the ice core record, it is not of the same amount as has happened over that time.

The cores from Dome C have evidence of 100 000 year cycles in temperature (Milankovitch Cycles – associated with changes in the eccentricity, axial tilt, and precession of the earth’s orbit) and gas analysis shows that carbon dioxide (oceanic control) and methane (terrestrial control) concentrations increased when average temperatures were high but decreased when average temperatures were low. However, prior to this there were 40 000 year cycles. The transition between these two periodicities is thought to have occurred at the limit of the available data (i.e. about a million years ago). Hence a future core covering an even longer time span will be important for investigating the transition and, particularly, the reason(s) why it occurred. A dramatic reduction in atmospheric carbon dioxide seems likely to have been involved.

Rapid climate change events also occur and 23 such events have been recorded between 110 kyr and 23 kyr BP. These events are much stronger in the Arctic than the Antarctic and the climate variability in cores from Greenland are much stronger than those from Antarctica. These Dansgaard-Oeschger events are linked to changes in the oceanic conveyor belt. Amongst other changes global temperatures are up by 10 ˚C, methane is up by 100-200 ppbv, and there is more snowfall during these warm events compared to the cold periods around them.

Eric Wolff co-chairs the International Partnership in Ice Core Science (IPICS) which aims (starting in 2012) to drill for two ice cores, with Dome A as perhaps the site for one of them. The data obtained from the million year old ice cores will help us to understand how the Earth works, and therefore to have better models to understand the future and hence determine the way we prepare for it.